It is known in motor vehicle suspension systems such as McPherson strut or other strut-type suspensions that side load forces on the strut can increase strut friction. Techniques for side load compensation such as seating the spring on an angle relative to the strut axis and pointing the bottom of the strut slightly outward from vertical have reduced side load friction from the strut.
FIG. 1 illustrates a prior art front suspension system assembly 10 including a strut and spring. The vehicle body includes a shock tower 12 comprising sheet metal of the vehicle within which is mounted a top mount assembly 14 through which the strut and spring are attached to the vehicle. The top mount assembly 14 comprises integrally molded rubber body 18 and rigid body members 22 and 24, typically made of stamped steel. Top mount assembly 14 is mounted to the body strut tower 12 by bolts 16 integrated into top mount assembly 14 placed through bolt holes in the strut tower 12. The top mount assembly 14 carries a bearing assembly 28 of a known type that is friction fit around the outside of the rubber 18 of the top mount assembly 14 and seats in top mount assembly 14 so that one side of bearing assembly is fixed relative to the top mount assembly and the strut tower. The second side of bearing assembly 28 freely rotates with respect to the first side of the bearing assembly, the top mount assembly and the strut tower.
The free rotating side of the bearing assembly 28 carries a composite spring seat 30 that is clearance fit to the outer diameter of the free rotating side of bearing assembly 28. The rubber isolator 32 sits within the spring seat 30 in between the spring 40 and the composite spring seat 30. Isolator 32 acts to prevent high frequency suspension noise in spring 40 from transferring to the spring seat 30 and the vehicle body. Jounce cup 34 is press fit within the inner diameter of rigid member 24 of the top mount assembly. Jounce bumper 36, comprising an elastomeric material such as urethane, and the plastic dust cover 38, are snapped within jounce cup 34 as shown. Affixed to the damper cylinder 48 in a known manner is the lower spring seat 42 including an isolator 44, made of an elastomeric material such as rubber, within which the spring 40 seats.
During assembly of the piston rod 46 to the top mount assembly 14, the spring 40 is compressed by a manufacturing aid of a known type until the end of piston rod 46 fits through opening 43 in the mount assembly. Next rate washer 11 and nut 13 are attached to the end of the piston rod 46 thereby holding the rod to the top mount assembly 14. The manufacturing aid then releases the spring, which forces piston rod 46 to its outer most position with respect to damper cylinder 48 and affects the fit of spring seat 30 to the outer diameter of the free rotating portion of bearing assembly 28.
On the lower end of damper cylinder 48 is attached mounting bracket 50 of a known type, which is affixed with bolts to knuckle 52 of wheel assembly 58. As shown in the Figure, the piston rod axis 60 is substantially coaxial with the axis of bearing assembly 28. Both the piston rod axis 60 and the axis of bearing assembly 28 are at an acute angle, for example, approximately 8.degree., to the quarter car steer axis 54, also referred to as the king pin axis, running between the center of the bearing assembly 28 and the center of the stud 56 of the wheel lower ball joint. An outline of the tire 55 is shown with respect to the suspension unit. As can be seen, the lower end of piston rod axis 60 is mounted at an outward vertical angle with respect to the true vertical axis 61.